1. Field of the Invention
The invention relates to liquid purification or separation by a process of precipitating the solids and dissolved material by chemical treatment with lime and carbon dioxide. More particularly this invention is concerned with the treatment of sanitary sewage using lime.
2. Description of the Prior Art
The use of chemicals, particularly lime, for treating sanitary sewage is not new. Lime has been used in and around outdoor toilets, livestock areas, and compost heaps for generations to reduce odors and mitigate certain forms of putrescence through its apparent biostatic effect. Furthermore, lime has also been employed as a precipitant and a flocculating aid in the clarification of turbid waters and for the removal of hardness and certain other chemical compounds in water.
In a conventional process for treating sanitary sewage it is customary to first remove the coarser and heavier insoluble matter by gravity settling and/or screening. After this preliminary treatment, the concentration of the insoluble matter remaining in the sewage is on the average around 150 to 500 parts per million colloidal or finely divided suspended solid matter. The finely divided suspended or colloidal matter remaining after this primary settling step includes simple and complex organic and inorganic compounds. Conventional sewage treatment consists of subjecting this material to biological activity wherein this organic material is decomposed to water, carbon dioxide and other innocuous inorganic compounds. The means of biological treatment consist of anaerobic digestion and aerobic digestion through a variety of methods including activated sludge and trickling filters. All biological means of treatment have a basic, inherent disadvantage, viz. they are susceptable to disruption of biological activity from a variety of sources, e.g., toxic materials, shock organic and hydraulic loadings and the like. At best they require a minimum of 4-8 hours of treatment, with variable, unpredictable, and frequently uncontrollable results. General cognizance of this led numerous investigators to apply chemical and/or physicochemical methods to the treatment of sanitary sewage in order to improve treatment efficiency and processability.
Thus sewage containing this residual soluble and colloidal suspension of organic matter has been subjected to flocculation by agitation with added alkali in an amount sufficient to give the sewage a pH in excess of seven. Frequently, other flocculating agents or a combination of flocculating agents are used, such as disclosed in U.S. Pat. No. 1,956,420, wherein ferric sulfate solution and lime are used together to give a floc which settles more rapidly.
The solids produced by the settling of the coagulation of suspended matter and the flocculation treatment contain a number of substances as a result of the treatment. U.S. Pat. No. 3,386,911 teaches that lime not only acts to increase flocculating and the formation of sludge, but also precipitates phosphorus when used at a pH of between 8 and 9. However that reference is directed toward a combined chemical and biological treatment of sewage for the removal of phosphorus to overcome water pollution resulting from the growth and subsequent decay of algae in receiving waters.
U.S. Pat. No. 3,423,309, teaches an improvement in the extraction of phosphorus from sewage by treatment with massive doses of chemical precipitants for phosphorus such as calcium oxides, alum, or iron salts by using a balanced and interrelated system utilizing an initial limited chemical precipitant, such as lime, for the removal of phosphorus by carefully controlling the pH. Other references relative to the treatment of sanitary sewage include U.S. Pat. No. 3,440,165 which teaches a sequential addition of calcium chloride and hydrated calcium oxide for the flocculating of solids. U.S. Pat. No. 3,440,166 describes a waste treatment process with recycling of the flocculating agents. Further, U.S. Pat. No. 3,442,498 teaches a combined waste treatment with a cement making process for making cement economically from the waste materials.
In all of the teachings cited above, lime is applied to remove or reduce the COD, the Biochemical Oxygen Demand (BOD) of phosphorus or residual suspended solids in fine dispersion in the sanitary sewage. Maximum purification of sewage on the basis of COD or BOD removal is obtained upon the addition of about 400 parts per million lime (see for example "Lime Precipitation at Sowford" by T. Jones, in the Journal of the Proceedings of Industrial Sewage Purification (1954), pages 395-402). Consequently, it was accepted in the prior art that a maximum of about 400 ppm lime addition to sewage is all that was required for treatment.
There are several disadvantages to these practices in the prior art. First, the addition of lime to sewage in concentrations at or less than 400 ppm requires a flocculation period after mixing in order to build a flocculate. This period can last from one-half hour to two hours or more. After the flocculation period, a settling period is required to permit the flocculate to separate from the liquid. Again, a period of one-half hour to more than two hours is required. After this, the effluent from the settling operation is treated with carbon dioxide to adjust the pH and to recover excess lime in solution and the flocculation and settling steps must be repeated for the separation of the calcium carbonate formed. This adds additional hours, and the large volume of sewage being treated requires large and costly tankage.
A second disadvantage to ordinary treatment of sewage with lime at levels of 400 ppm or less is found in the type of sludge that is formed. Not only does it require a long time to form and settle, but it does not compact well, producing a watery sludge with solids content less than 1% without special thickening treatment that increases process complexity. This means that large quantities of sludge must be handled relative to the total sewage flow to the treatment process. Furthermore, these sludges, when they contain organic or biological matter, are difficult to dewater to higher solids content in the course of disposing of the sludge. Sludge dewatering is most desirably accomplished by vacuum filtration and these sludges, to be handled in a practical manner on rotary vacuum filters must be conditioned with various types of added chemicals. The improvement by conditioning the sludge is frequently marginal and usually costly.
Yet another disadvantage of the ordinary methods of treating sewage with lime at dosages at or below 400 ppm lime lies in the fact that the amount applied is "just the right amount" to attain maximum COD or BOD removal, based upon a statistical average value of these parameters for the sewage being treated. Since the values of these parameters cycle throughout the day, they obviously vary about the average value to considerable extent. Since the lime dosage does not take this variation into consideration, high values of COD, or BOD, would not receive the same amount of treatment as the average values, since the lime dosage is marginal in this respect. Thus, the effluent from the treatment process would show cyclic values of COD and BOD instead of a uniform low, or "floor" value of these parameters on a continuous basis.
The present invention overcomes the disadvantages of the prior art by providing a process that adds lime to raw, sanitary sewage in amounts far in excess of that previously used and thereby produces sludge that settles extremely rapidly without a separate flocculation step, compacts on settling to solids contents in excess of 3% solids by weight, dewaters by vacuum filtration with filter cake yields at least three times those achieved by prior art of chemical or biological treatment, and produces a clear, treated effluent at a consistent, continuous "floor" level of COD and BOD that is independent of variation of the feed values of these parameters up to two to three times their average values.